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We present arcsecond resolution observations from the VLA 1 with full Stokes polarimetry of the ground-state satellite line of the hydroxyl molecule (OH) at 1720.53 MHz ( 2 Π 3 2 , J = 3 2 , F = 2 → 1) toward three Galactic supernova remnants: W 28, W 44, and IC 443. The total number of individual OH(1720 MHz) "spots" we detect in each of these three remnants is 41, 25 and 6, respectively. The OH(1720 MHz) features appear to lie along the edge of radio continuum emission from the supernova remnants, but are displaced behind the leading edge of the shock as traced by the synchrotron emission. The brightness temperatures of the OH(1720 MHz) emission features range from 2×10 4 to 10 8 K, convincingly demonstrating the maser nature of the OH(1720 MHz) features. We argue that the partially resolved angular diameters that we measure for the masers are neither intrinsic sizes nor scattering disks, but result from a blend of several unresolved maser features near the same velocity. Thus our computed brightness temperatures are lower limits to the true values. The characteristic antisymmetric "S" profile, indicative of Zeeman splitting in the weak-field case, is identified in the Stokes V spectrum of several of the brighter maser spots. The derived line-of-sight magnetic fields are of order 0.2 mG and are remarkably constant in both direction and in magnitude over regions several parsecs apart. These are the first measurements of post-shock magnetic fields in supernova remnants and demonstrate the importance of magnetic pressure in these molecular shocks. The velocity dispersion of the maser features is typically less than a few km s −1 , and except in the special case of W 28, the mean maser velocity is equal to the systemic velocity of the remnant. We suggest that the maximum amplification of the maser transition will occur when the acceleration produced by the shock is transverse to the line of sight. Additional support for this point comes from the location of the masers in IC 443, and molecular observations which allow the shock geometry to be determined. All of our observations are consistent with a model in which the OH(1720 MHz) is collisionally excited by H 2 molecules in the postshock gas heated by a non-dissociative shock. Finally, we end with a discussion of the importance of supernova remnants with OH(1720 MHz) maser emission as promising candidates to conduct high energy searches for the sites of cosmic ray acceleration. Subject headings: supernova remnants -masers -ISM: magnetic fields -ISM: individual(W 28, W 44, IC 443) 1 The Very Large Array (VLA) is operated by NRAO, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

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Using the Hubble Space Telescope (HST ), we have carried out a survey of candidate preplanetary nebulae ( PPNs). We report here our discoveries of objects having well-resolved geometric structures, and we use the large sample of PPNs now imaged with HST (including previously studied objects in this class) to devise a comprehensive morphological classification system for this category of objects. The wide variety of aspherical morphologies which we have found for PPNs are qualitatively similar to those found for young planetary nebulae ( PNs) in previous surveys. We also find prominent halos surrounding the central aspherical shapes in many of our objects; these are direct signatures of the undisturbed circumstellar envelopes of the progenitor AGB stars. Although the majority of these have surface brightness distributions consistent with a constant mass-loss rate with a constant expansion velocity, there are also examples of objects with varying mass-loss rates. As in our surveys of young PNs, we find no round PPNs. The similarities in morphologies between our survey objects and young PNs supports the view that the former are the progenitors of aspherical PNs. This suggests that the primary shaping of a PN does not occur during the PN phase via the fast radiative wind of the hot central star, but significantly earlier in its evolution.